This invention relates to sensing devices which are adapted to provide an electrical indication of a sensed parameter and, particularly, relates to sensing devices arranged for mounting in the wall of a chamber to provide at a terminal outside the chamber an electrical indication of a parameter sensed within the chamber.
In automobile engines increasing use is now made of devices for sensing temperatures and pressures in various regions, the outputs of these devices being used to drive a display or to control a particular engine function. Taking the example of temperature sensing devices, these may be electro-mechanical in nature--such as thermostats incorporating a bimetallic element--or electronic, examples of the latter being NTC devices. In both cases the conventional method of construction involves placing the sensor element in a brass housing and fitting an insulating cap to the housing with suitable O-ring seals. The insulating cap is formed with the appropriate electrical terminals which connect internally of the housing with contacts on the sensor element. Assembly of the device is completed by deforming the housing around the periphery of the cap to hold the cap in place. Depending upon the intended use of the sensing device, the housing can be formed--for example--with an exterior thread and an integral hexagonal portion enabling the device to be screwed into a threaded aperture, or with a smooth exterior surface terminating in a shoulder so that the device can be pushed into an aperture to a depth governed by the shoulder, the aperture often carrying a grommet to ensure an efficient seal.
Sensing devices constructed in this manner are expensive, primarily because of the relatively high cost of turned brass parts but also because of the time-consuming assembly procedure. In performance, moreover, this type of device has important disadvantages. Thus, as the temperature sensor element is enclosed within a brass housing, the thermal response time is necessarily slow. Attempts have been made to alleviate this difficulty by using aluminum rather than brass housings, but without expensive anodising techniques the aluminum is not sufficiently resistant to corrosion. In general the difficulty is dealt with by increasing the sensitivity of the sensor element to compensate for the time lag, but this of course increases the cost of the device. A further important disadvantage lies in the fact that a brass housing--particularly if formed as is customary with a hexagonal nut portion for screwing the device into a threaded aperture--represents a significant heat sink drawing heat from the sensor element. The temperature actually sensed is therefore not that of the chamber in which the sensor element is placed but is rather an effective temperature lying between the actual temperature and the exterior temperature to which the brass housing is subjected.
A temperature sensor having a moulded plastics body has been proposed and this goes some way to overcoming these difficulties. Reference is directed in this connection to UK Patent Application Ser. No. 2,035,553A which discloses a temperature sensor having an injection moulded plastics body into which the sensor element and terminals are inserted and then secured in place by a sealing compound.
It is an object of this invention to provide a more economic method of manufacturing sensing devices and in particular to reduce the number of manufacturing operations. It is further object to provide a sensing device with an improved performance. It is yet a further object to provide a sensing device which is robust.
Accordingly, the present invention consists, in one aspect, in a method of manufacturing a sensing device adapted for mounting in a chamber wall to sense a parameter within the chamber and to provide at one or more terminals outside the chamber an electrical indication of said parameter, comprising the steps of forming a sub-assembly of one or more terminals with a sensor element including contact surfaces, the or each terminal being disposed in electrical contact with a respective contact surface of the sensor element; and moulding an integral plastics body around a part of the or each terminal and at least a part of the sensor element to secure the or each terminal to the sensor element, the moulding being so arranged that the resultant plastics body is outwardly shaped for engagement within an aperture formed in the chamber wall.
Advantageously, the moulding step comprises injection moulding the plastics body in a mould containing said sub-assembly as an insert.
In a further aspect the present invention consists in a sensing device manufactured in accordance with a method as set out above.